A chuck gripping a work in a leading end of a main spindle is installed to a machine tool such as a lathe turning machine. In the automated lathe turning machine, the main spindle is constructed by a hollow shaft, and work gripping and releasing operation of the chuck is carried out by movement in an axial direction of an operation rod which is inserted into a hollow hole. A chuck cylinder moving the operation rod is attached to a rear end of the main spindle. Since a diameter of a portion which is gripped by the chuck is different according to a kind of the work (hereinafter, refer to as “product type”), a position of a claw of the chuck when gripping the work is different according to the product type. Therefore, a position in an axial direction of the operation rod when gripping the work is different according to the product type.
A hydraulic pressure which is necessary for the chuck to grip the work with a sufficient force is supplied to the chuck cylinder. In the case that the hydraulic pressure in a work gripping direction is supplied to the chuck cylinder in a state in which the work is inserted into the chuck, the operation rod moves in the work gripping direction, and the movement in the axial direction of the operation rod stops when the claw of the chuck grips the work with the sufficient force.
However, in the case that the claw comes into collision with something and stops before the claw grips the work, for example, due to attachment of foreign material to the work or supply of different diameter work, the work can not be gripped by the sufficient force. In order to avoid this problem, a sensor detecting a moving position in the axial direction of the operation rod is provided.
More specifically, a dog (a detection portion) moving integrally with the operation rod is provided, and a sensor such as a proximity sensor is placed at a position where the dog is positioned when the work of the product type to be processed is gripped. The hydraulic pressure in the work gripping direction is supplied to the chuck cylinder in this state, and the process of the work is started by confirming that the sensor outputs the detection signal when the movement of the operation rod stops and determining that the work is normally gripped.
In the device which confirms the gripping of the work with the proximity sensor, it is necessary to change a set position of the sensor every change of the work product type to be processed. In a general-purpose machine tool in which the product type of the work is changed frequently, it is necessary for an operator to manually set the position of the sensor every change of the product type. As a result, a load of the operator is great, and it takes a long time to change the setup of the machines when the product type is changed. Further, there is the danger that a defect product is generated and the tool is broken due to setting error, and the skill of the operator is necessary.
In order to avoid the problem, patent documents 1 and 2 have proposed to structure such that the position of the operation rod when the work is gripped can be set, for example, on the basis of a numerical value input from an operator control panel, by the provision of a stroke sensor which can continuously detect the position in the axial direction of the operation rod. The patent document 1 has also proposed to automatically set the position of the operation rod when a standard work is gripped by the chuck. According to the means, it is not necessary to carry out a complicated and skilled work of finely adjust the position of the sensor by hand. Further, it is possible to prevent the setting error to some extent by displaying the set position on the display.